1. Field of the Invention
The present invention relates to a mobile communication system, a mobile communication terminal, a cell search method and a program for use therewith, and more particularly to a cell search method in a W-CDMA (Wideband-Code Division Multiple Access) mobile communication system.
2. Description of the Prior Art
An asynchronous W-CDMA communication system is one of the communication systems as defined in accordance with the IMT2000 (International Mobile Telecommunication-2000).
In this asynchronous system, a scrambling code is assigned to each cell, and a mobile communication terminal employs a cell search technique of three stages to detect a cell to which the self terminal belongs.
FIG. 7 shows the formats of synchronous channels used for the cell search. FIG. 7A shows one super-frame consisting of 72 frames, its period being 720 ms. FIG. 7B shows one frame consisting of fifteen slots, its period being 10 ms.
FIG. 7C shows p-CCPCH (primary-Common Control Physical Channel) of nine symbols, and one slot consisting of P-SCH (Primary-Synchronization Channel) and S-SCH (Secondary-Synchronization Channel) of one symbol, its period being 0.667 ms, and one symbol being composed of 256 chips.
FIG. 7D shows CPICH (Common Pilot Channel) for use to recognize the scrambling code, its period being 0.667 ms.
There area total, of 512 kinds of scrambling codes assignable to the cell, which are divided into 64 kinds of code groups. Each code group has eight kinds of scrambling codes (64×8=512).
To detect the cell to which the self terminal belongs, the mobile communication terminal makes the synchronization establishment of P-SCH at a first stage, as shown in FIG. 6. Since P-SCH has only one code that is predetermined for all the base stations, it detects the slot timing through inverse spreading using that code and makes the slot synchronization establishment (step S11 in FIG. 6).
A second stage involves making the synchronization establishment of S-SCH in accordance with a slot timing detected at the above-mentioned first stage. Since S-SCH has 16 kinds of code in a unit of slot, one code group being decided from 64 kinds of code groups in combinations thereof, inverse spreading is made round a total of 16 kinds of codes in accordance with the slot timing, detecting a frame timing, and making the frame synchronization establishment (step S12 in FIG. 6). This synchronization establishment requires inverse spreading of three slots (2 ms) at minimum, and 15 slots (10 ms) at maximum (e.g., refer to Patent Document 1).
A third stage involves making the inverse spreading for CPICH round eight scrambling codes belonging to the code group proved at the second stage in accordance with the frame timing detected at the second stage and recognizing the scrambling code assigned to the cell (step S13 in FIG. 6).
The details of the frame configuration, slot configuration, SCH, and scrambling code are arranged as the standard recommendation in 3GPP (Third Generation Partnership Project).
[Patent Document 1]
Japanese Patent Laid-Open No. 2003-283371
In the conventional cell search method, since the processes of the first stage (slot synchronization establishment), the second stage (frame synchronization establishment) and the third stage (scrambling code recognition) should be performed in due order, there is a problem that the cell search time is longer. Herein, the second stage requires a time of 2 ms at minimum and 10 ms at maximum.
However, if the cell search process is completed in a shorter time, there is a merit that the service time of battery for the mobile communication terminal is increased, and another process is performed for that time.